The invention is described below with reference to an IR sensor for IR-absorbing gases, but can also be used for other purposes, as will be explained hereinbelow.
Such a sensor, which is configured as a gas sensor, is known, for example, from U.S. Pat. No. 5,081,998 A. An IR radiation source is provided therein, which acts upon a total of four detectors by way of a filter arrangement. The filter arrangement has two filters having different pass characteristics. A first filter has a passband for IR radiation that is absorbed by CO2. That filter is therefore also referred to as a “CO2 filter”. The detectors arranged downstream are designated CO2 detectors. The other filter has a passband different therefrom which serves for determining a reference quantity. The detectors arranged downstream of that reference filter are referred to as reference detectors. Between the IR source and the two filters there is arranged a third filter which is referred to as a natural density filter and overlaps half of the first filter and half of the second filter. Accordingly, one of the two CO2 detectors and one of the reference detectors receives only IR radiation that has passed both through the natural density filter and through either the CO2 filter or the reference filter. In the evaluating device, the difference of the output signals of the two CO2 detectors and the difference of the two reference detectors is formed. The two differences are then divided by one another. Such a CO2 sensor is required, for example, for determining CO2 in a patient's breath so as to be better able to monitor the patient during anaesthesia.
Another field of use for gas sensors, especially CO2 sensors, is described in U.S. Pat. No. 6,369,716 B1. The CO2 sensor therein is used for determining the CO2 content (carbon dioxide content) in a room in order to be able to control the room climate with the aid of that measured quantity.
The CO2 concentration in a room should be between 800 ppm and 1200 ppm, because higher concentrations can give rise to fatigue symptoms. The natural concentration in built-up areas is normally about 400 ppm. With the aid of a CO2 sensor it is possible to determine how much fresh air needs to be supplied in order to achieve a desired CO2 concentration. The same considerations apply also to other gases for which a specific content should not be exceeded, for example CO (carbon monoxide) or the like.
As mentioned above, the invention is described below with reference to the measurement of CO2, but it is also suitable for use for other gases.
One way of measuring CO2 in the air is based on gas-phase-based sensors in which non-dispersive infrared spectroscopy (NDIR) is used. Such a method of determining the CO2 content proceeds on the basis that CO2 absorbs infrared radiation, that is to say the content of IR radiation in a specific, narrowly defined wavelength range is a quantity that can be used for determining the CO2 concentration.
A disadvantage of such sensors is that they have a relatively high power requirement. The arrangement known from U.S. Pat. No. 5,081,998 A requires a source of radiation which, in any case for prolonged use, makes it unsuitable for battery-operated use. Furthermore, such an IR source generally requires a certain heating-up period, so that without a degree of prior preparation it is not always possible to carry out measurements when desired.